Claudia Kienzle /
01.19.2005 12:00 AM
UD Proposal Draws Fire
Initial tests show serious interference problems
One of the industry's biggest battles with federal bureaucrats in recent years is taking shape in 2005 as broadcasters weigh in with their concerns about the FCC's proposal to allow unlicensed devices (UDs), such as wireless computer networks, to operate in the vacant "white spaces" of the broadcast television spectrum.
Last May, the commission floated the idea in a Notice of Public Rulemaking (NPRM). By the Nov. 30 filing deadline, it had received 147 thoughtful, passionate replies for and against the proposed ruling. The FCC granted a request from the Consumer Electronics Association (CEA) to extend the deadline for reply comments to Jan. 31 to give the organization time to complete field tests it is performing and to give it time to respond to other submitted comments.
"There's a huge potential upside here," said Michael Petricone, CEA's vice president for technology policy. "You're talking about expanding access to wireless broadband networks, which offer huge marketplace and consumer benefits. But this needs to be done in such a way that doesn't interfere with the broadcast signal or the DTV transition."
Maximum Service Television (MSTV), broadcasters' Washington, D.C. representative on technical issues, however, is extremely concerned about the proposed ruling. "The FCC appears to have two competing regulatory objectives--fostering an environment favorable to wireless broadband, while promoting the DTV transition. But, DTV should be their primary concern," said MSTV's president David Donovan.
"The bottom line is that 73 million analog sets receiving off-air signals, as well as those receiving TV via cable or satellite, could be affected by interference from UDs. DTV sets could display a frozen or blank screen prompting consumers to perceive DTV as an unreliable service," he said. "At this critical point in the DTV transition, the FCC has decided to play 'interference roulette.'"
In joint comments filed by MSTV and NAB, MSTV disclosed results of extensive testing by the Communications Research Center (CRC) Canada, including a spectrum analysis study that backed MSTV's claims with hard data.
Prior to testing, "we asked the FCC to clarify the specifics, such as the modulation schemes, power levels, types of wireless services and other technical parameters being considered, but they gave us extremely vague answers," said Donovan. "This made real-world testing very difficult to do."
The CRC study found "there is a significant desensitization of the [DTV and NTSC] TV receiver as far away as 24 meters... and proportional to the UD's signal bandwidth and distance... even if there is an intervening wall... between the UDs and receiver, as in an office, hotel, or apartment."
The CRC study also showed that indoor UD operation would interfere with cable and satellite reception. CRC transmitted a 100-mW wideband signal through a Silver Sensor antenna with approximately -5dB gain and measured its effect on an RG-6 double-shielded cable and an RG-59 single-shielded cable; both picked up significant interference from portable devices in use nearby.
The NCTA cited an additional risk to cable operations in its comments: "While broadcast signals are generally received at the cable headend within the Grade B contour... and protected under the proposed rules, there are many instances where broadcast signals are received from locations outside of the Grade B contour, many of which are deemed 'must carry.' Under the proposed rules, UDs could transmit on channels used for receipt of distant broadcast TV signals, increasing the likelihood of interference..."
LIVE COULD GO DEAD
Live television productions involving hundreds of wireless microphones could also suffer from interference, should UDs be permitted into the broadcast spectrum without proper interference mitigation techniques. At any live news event or live telecast, "you already have intense competition for spectrum for wireless microphones, often on a moment's notice. Without this spectrum, we go back to running mic cables or forgoing live telecasts altogether," said Edgar C. Reihl, principal engineer for Shure, Inc.
"If UDs cause harmful interference on the same channel where somebody is singing the national anthem for the Super Bowl, that broadcast is going to go off the air," Reihl said. To prove this, Shure applied to the FCC for a Part 5 Experimental License to conduct dynamic real-world tests of wireless microphone operation in the presence of co-channel interference.
"We frequency-translated Wi-Fi devices down into the UHF TV band, band-limited them to 6 MHz and amplified them to 100 mW [the power level the commission proposed]... and the result was significant interference," said Reihl.
Shure made three recommendations for mitigating UD interference. 1) Identify two VHF and four UHF TV channels to be exempt from UD operation; 2) require UD to employ spectrum sensing/dynamic frequency selection techniques in a distributed, cognitive fashion; and 3) implement a "smart beacon" system operating on one of the vacant TV channels being used by the wireless microphone system and transmit information concerning the TV channels they are using.
FINDING SMART SOLUTIONS
Carl Stevenson, chairman of the newly formed IEEE 802.22 working group, said "We are developing a new IEEE 802 series standard that will enable license-exempt devices, such as Wireless Regional Area Network [WRAN] devices, to operate in the VHF/UHF broadcast bands [between 54 MHz and 862 MHz] on a strictly noninterfering basis through the use of cognitive smart radio techniques.
"We in IEEE 802 have been working in close cooperation with MSTV, Fox, the IEEE Broadcast Technology Society as well as the CEA, the NPSTC [National Public Safety Telecommunications Council] and Shure for months," Stevenson said. "Their representatives voted in favor of the IEEE 802 proposals, and expressed a desire to find adequate solutions, such as cognitive radio techniques, that enable wireless computer networks to use television broadcast's spectrum in a way that avoids causing interference to the incumbents.
"The FCC can and should enact appropriate rules dictating the behavior of devices to ensure that only those that avoid interfering would ever enter the marketplace."
CAUSE FOR CONCERN
But MSTV's Donovan is afraid that the proposal follows the proverbial "letting the horse out of the barn" mistake.
"In its NPRM, the commission said, 'If there is interference from UDs, broadcasters will take precedence.' But that guarantee is meaningless because there will already be millions of wireless devices in the hands of consumers, and it will be impossible to retrieve them or identify which one is causing interference with someone's TV set."
"This initiative is at the proposal stage," said Bruce Romano, associate legal chief for the FCC's Office of Technology and Engineering. "In the next several months, the commission can adopt some or all of the proposed provisions or some new variation suggested by the comments, or it could determine that the proposals should be scrapped altogether, although it's fairly uncommon for the commission to walk away from a proposal. More often, the commission uses the information it receives to adapt its proposals to respond to specific concerns.
"But an initiative like this is big news, so we have given the public ample opportunity to respond. This is not a done deal, but broadcasters have a reason to be paying attention because obviously we're thinking about it. We saw sufficient merits to issue an NPRM and invite feedback from affected parties to make a better, more reasoned decision."